2008 IEEE Asia-Pacific Services Computing Conference

Zigbee - Research into integrated Real-Time Location Systems
Chung-Hsin Liu1, Chih-Chieh Fan2,
1 2

Department of Computer Science

Department of Information Management

Chinese Culture University, Taipei, Taiwan, ROC

Abstract-Wireless sensor networks (WSN) have evolved quickly in recent years, leading to systems such as RF and Zigbee, which are commonly adopted in lights, curfew, elderly – nursing. However, RF and Zigbee system are seldom integrated effectively. This study presents a system built like WebNet, expressed in terms of UML. The proposed system behaves like Zigbee with NS2, since it combines RF with a Real-Time Location System. It can be applied to equipment and staff tracking, and is effective for equipment and asset management. The wireless transmission condition of Zigbee is simulated by NS2, and compared with the actual result. In addition to Zigbee Gateway Interface, the indoor location system is formed to from the Zigbee badge and Zigbee Location Node, and transmits information about the current situation and relative position of the equipment to WebNet through an Ethernet connection. Index Terms—WSN, Zigbee, NS2, Real-Time Location System

I. INTRODUCTION The continuous improvement of sensor skill and wireless communication is encouraging wireless sensor networking. Wireless sensor networks are adopted in many ways, including industrial control, hotels, environmental monitoring and digital home [1]. Developing sensors involves solving many problems, including location, safety and power waste. This study mainly concerns about position location. The mostly common location method is GPS [2, 3]. GPS is easily blocked by buildings, to power-intensive; cannot be adopting indoors, and only works in small spaces. Additionally, GPS is not accurate enough for many reasons. Moreover, outdoor between the signal of wireless network needs to cover a whole region, while the signal of an indoor environment is often cut off by compartment blocking. Therefore, the network for real location system should be designed by different system ranges. A real location system houses many devices. The capacity is also important, which is why the latest Zigbee is adopted. Zigbee is low-cost, low-power and small, making it appropriate for

location systems.

II. POSITIONING TECHNOLOGY The common indoor position parameters include TOA(Time of Arrival), TDOA(Time difference of Arrival), AOA(Angle of Arrival) and RSS(Received Signal Strength) [4]. Many receivers are adopted to locate the position of a user. This study adopts NNSS-AVG Nearest Neighbor in the Signal Strength of Pattern Matching algorithm, combined with event-driven and path analysis to reinforce the accuracy of the result. The following location methods are adopted.

A. Trilateration The traditional method is Trilateration, which involves calculating the actual coordinates by using the distance from the target to the referring point. In Fig. 1(a) below, A, B, C are three points, and rA denotes the distance from point A traveled by the signal. Circle A is drawn from the center A and radius B. Circles B and C are also drawn similarly, thus determining the position of the target from the intersection of three circles.

short-distance wireless transmission. Additionally, its two-way communication function is also suitable for indoor
978-0-7695-3473-2/08 $25.00 2008 IEEE DOI 10.1109/APSCC.2008.96 942

focused and three–dimensional on the front of the antenna, but reduces in size at the back and in other directions of before the antenna, as indicated in Fig.3.

(a) Figure 1 Trilateration


The actual situation has some errors and these make errors of location. Consider the case in Fig.1 (b), where the three circles do not make an intersection. The Maximum likelihood method is adopted to predict the position of the target in Figure 2. [4, 5] RF signals are often adopted on sensor devices of office gates. To pass through the gates, a visitor stands in the front of the gate. The sensor senses the visitor’s presence, and Figure 3 Patch antenna signal character

( X X A )2 + (Y Y A )2 rA
σx,y =

+ +

the door opens. This pattern can be adopted to examine and increase the location accuracy.

( X X B )2 + (Y YB )2 rB ( X X C )2 + (Y YC )2 rC

C. Path Analysis To increase the location accuracy, this study also adopts

Figure 2 Maximum likelihood method Equation

The terms rA, rB, rC denote the distances from A, B and C, respectively, and finally telling the position by minimum

path analysis, which can neglect unreasonable location results. For instance, target X and target Y in Fig.4, if the target C should be between A and B through time ΔT (the time taken to move to A or B), then X and Y can be neglected.

σx, y.
Shortcomings: Trilateration is a simple concept, but needs at least three points to measure the location. Due to

the environment of the wireless sensor network, many sensors are randomly placed, and are thus unable to locate. Adding more sensors increases the cost. Therefore, this study adopts RF Event-Driven to solve this problem.

B. RF (Event-Driven) Zigbee sometimes cannot accurate detect the location, because signals are weak, cut off or doubled by compartments. To RF technology and antenna are adopted to solve this problem, and reduce power consumption in Zigbee. The source of signals is detected by a Patch antenna, which can offer obvious patterns. The signal is D. Pattern matching algorithm Locate based on signal strength is called Pattern Matching [4]. Rather than measuring the distance between Figure 4 Path Analysis


a target and a point, Pattern Matching compares the strength patterns to the training location patterns in the data file. Each signal strength is different, and can effectively reduce the error caused by unstable signals. The process can generally be divided into two stages, namely training and positioning. Gathering more training positions and signal samples leads to a more accurate result. The location process has two parts, namely off-line and real-time. The main task of the off-line step is to obtain the signal strength from the receiver, then adding it to a database. The real-time step obtains the signal strength from the receiver. The Signal Strength is calculated by the NNSS-AVG Nearest Neighbor method, as in Fig.5.

mange and maintains it easily. WebNet has three parts, namely Device Management, User Management and Accounting Management. The network and users are classified and managed in terms of organizations and locations. The UML Use-Case Diagram in Figure 6 denotes the relationship among location, organization, equipment and users [7]. Supervisor is easily monitor each of equipment status By WebNet System .Figure 7 explains the relationships among the relative positions of each device.

This algorithm is adopted to reduce the number of errors resulting from signal moving, and is based on NNSS(Nearest Neighbor in Signal Strength). Each

geometric distance of training positions and signal samples is calculated from the location stage. The K minimum training positions are chosen, and then averaged to obtain the result.

Figure 6 UML Use-Case Diagram for WebNet system

Figure 5 NNSS-AVG Flow Chart III. WEBNET SYSTEM A. WebNet system construction WebNet is a middleware to develop for managing wireless network equipment, and we adapted Fontal [6] Zigbee hardware for implementation. Zigbee Extension Gateway sends information about the Zigbee situation and badge position to the WebNet system, enabling users to The WebNet system integrates wireless network structure combines Zigbee and RFID. Besides to the Zigbee Gateway Interface, the main indoor location system for Figure 7 Relative Positions of Each Equipment


WebNet comprises a Zigbee badge and Zigbee Location Node. These components send information to the WebNet system to through the Ethernet and Zigbee, thus ensuring that WebNet knows the current situation and position. The WebNet system adopts UML Class Diagrams to describe the system structure, as seen in Figure 8. The

Figure 9 Network Subsystem






Coordinator and Bridge. It is the central system of the Zigbee network, and sends Zigbee signals to the WebNet Position Server through Ethernet, while also expanding the Zigbee badge Node and extending signals. The Zigbee Location Node acts as a Router, offering Beacon needs via the Zigbee communication protocol.

D. Location Subsystem

Location signal procedure Zigbee location signals are operated by an IEEE 802.15.4 Beacon. The Zigbee badge broadcasts a Beacon Request message after being woken up, and then the Location Node answers the Beacon after receiving the request. Zigbee accumulates all the Beacons, records them, calculates the signal strength, and sent this to the WebNet Figure 8 UML Class Diagrams for WebNet system Server through Zigbee Gateway for further calculation. Fig.10 shows the procedure based on the UML Sequence B. Zigbee Real-Time Location System The Zigbee Real-Time Location System [6] comprises the Network and Position Subsystems, which are described in detail below. Diagram.

C. Network Subsystem The Network Subsystem (Figure 9) comprises the Zigbee Gateway and Zigbee Location Node, and is described in detail below:

Figure 10 UML Sequence Diagram for the procedure based

IV. NS2 IMITATING ZIGBEE LOCATION SYSTEM As indicated in graphic 11(Fig.11.), NS2 was adopted


to imitate the situation of location system [8, 9], and compare Zigbee added to RF functions and analysis the result. Four point positions were simulated: n1(200, 300),

The average error using Zigbee added to RF was 3.35cm for n0(200,500), the average error for n5(450,98) was 2.78cm and the average error for n7(700,141) was 0.98cm. Figure 13 shows the result of n0(200,500).

n3(400, 300), n4(600, 300), n5(600, 500). The coordinates of
targets are n0(200, 500), n5(450, 98), n7(700, 141). Movements n0, n5, n7 via n1(200, 300), n3(400, 300), n4(600, 300) and n5(600, 500) were simulated.

Figure 13

Figure 14 compares the normal Zigbee and the research (Zigbee + RF), showing an accuracy of up to 98.5% testing in the range of 5m of the indoor Figure 11 NS2 imitating Zigbee Location System
100 98 98.5 98.34 97.73




96 94 92 90 88 86 1M 2M 3M 4M 5M 93.87 93.23 92.45

We got 8 coordinates (S1-S8) and got the average error 13.35cm for n0(200,500) by NNSS-AVG method, the average error for n5(450,98) was 23.78cm, the average error for n7(700,141) was 9.98cm, like the numbers for



n0(200,500) as shown in Fig.12.

Figure 14 The Zigbee combines RF can effectively increase the accurate rate through this conclusion. This lab doesn’t have too much compartments, so it’s high accurate. The accurate will be increased after adding location node in the actual environment like housewares because there are more obstacles. This study indicates that Zigbee combined with RF can effectively increase the location accuracy. The accuracy is high because the area has few compartments.



The authors would like to thank Dr. Tsai, Dwen-Ren (Chairman of the computer science) and Dr. Chen, Yih-Young (Dean's of the College of Engineering), for supporting this research.


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